First, because Microsoft is clearly buying these search share gains through uneconomic distribution deals that negate the benefit of the growth.
How do we know that?
Because Microsoft's financial disclosures suggests that Microsoft is paying as much to gain every point of share as it is generating in revenue from each point of share. Put differently, it appears Microsoft has a 0% gross margin on its search market growth of the past year: For every $1 of new revenue it generates, it spends $1 in traffic acquisition costs.
I found that whole article rather negative and short sighted, but worth a read.
I'm a lot more comfortable with the idea that Bing's growth will come, but it'll be over time. Too many people have become used to using Google, and more often than not, they will stay, however, user dissatisfaction will drive share away from Google. In some respects, it's no longer about whether the results are good enough, it's about brand building, and that takes time.
I'd like to bet against it, too. But I think dissatisfaction will drive away users. With being the leader comes alot of shennagins. g's serp's are full of useless results (i.e. scraped sites with redirects, fake results page with no relavence, etc..) These types of issues make me wish there was a real alternative. I don't consider m to be a real alternative - at least not for the long haul. m will eventually cannibalize b once the think of a better name - and they are not transparent or neutral.